Plasticized polyvinyl chloride (P-PVC) is useful in numerous applications such as film, geomembranes, hose and tubing, and upholstery. Its most important attributes are toughness (high tensile strength), flexibility (low modulus), and elastic recovery (essentially no yield point).
The elastic recovery of a polymer, particularly, can be very important to its end-use application. Thus, a hose should be resistant to kinking when it is bent over a short radius, and it should recover (straighten out) after it is subjected to a permanent set. In the film area, especially in food packaging, e.g., meat wrap, the film should bounce back after it is depressed with a finger or other object.
P-PVC has several shortcomings, however. For example, it has a limited use temperature. P-PVC becomes brittle below 0.degree. C. because of its high glass transition temperature (about minus 10.degree. C.) and becomes too soft above 70.degree. C. because of its low softening point. The plasticizers conventionally used in P-PVC tend to migrate to the surface of the polymer upon aging. This migration results in an increase in modulus and detracts from the long term performance of the P-PVC. Further, P-PVC releases toxic gases when exposed to fire and raises environmental concerns with respect to its disposal. Both the recycling and incineration of P-PVC are difficult undertakings.
The properties of P-PVC, in particular its modulus, vary with the amount of added plasticizer. As this amount is increased, the modulus of the resulting blend decreases. Consequently, P-PVC is classified according to its hardness on an A-scale as per ASTM D-2240. The hardness of the P-PVC on this scale can vary from as low as 40 for very low moduli to as high as 90 for high moduli.
In contrast, polyethylene has a very low glass transition temperature (below minus 100.degree. C.) and a softening point above 70.degree. C. It can be recycled with less difficulty than P-PVC and does not release toxic gases on incineration. Since the flexibility of a polyethylene product is related to its density, a plasticizer is not required for this purpose. By varying the density of the polyethylene, the hardness of any P-PVC can be achieved.
In order to provide a resin composition having physical properties comparable to P-PVC, then, the properties of toughness and recovery have to be addressed.